Recent studies of memory phenomena in animals have emphasized the importance of retrieval deficits as sources of retention loss, but a number of aspects of the retrieval processes are not yet understood. The proposed research attempts to modify memory in several ways in order to provide information about retrieval mechanisms. One series of studies will investigate the conditions under which old (reactivated) memory is modifiable by amnestic treatments, and will compare both old and new memories with respect to these changes. A second area of study focuses on the potential role of endogenous hormones as cues which support memory retrieval. Whether internal cues can function to reintegrate earlier aversive experiences and thereby lead to new learning in the presence of neutral exteroceptive stimuli will be examined. Experiments also will be conducted to determine the retention of attributes of stimuli present at training and to investigate whether variations in encoding context can attenuate forgetting. The research is designed to provide basic information on the plasticity of memory as well as the nature of retrieval processes.